Bypass controlled regeneration of NOx adsorbers

ABSTRACT

In a method and apparatus for regenerating a lean NOx adsorber, the NOx adsorber treats exhaust gases created during the combustion of gaseous fuels in general. A bypass line maintains a target regeneration flow of exhaust gas through the NOx adsorber during regeneration regardless of operating demands on the engine. Closed-loop and open-loop control are employed. The closed-loop control employs sensors that determine properties of the exhaust gas during regeneration, and the controller uses those properties to provide an efficient regeneration cycle. A regeneration map is also provided that uses creation of in-cylinder regeneration conditions for the exhaust gas in combination with in-line regeneration conditions for the exhaust gas.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of International Application No.PCT/CA2003/001467, having an international filing date of Oct. 2, 2003,entitled “Bypass Controlled Regeneration Of NOx Adsorbers”.International Application No. PCT/CA2003/001467 claimed prioritybenefits, in turn, from Canadian Patent Application No. 2,406,386 filedOct. 2, 2002, and Canadian Patent Application No. 2,422,188 filed Mar.14, 2003. International Application No. PCT/CA2003/001467 is also herebyincorporated by reference herein in its entirety.

FIELD OF THE INVENTION

This invention relates to methods and apparatus for regenerating NOxabsorbers used in association with internal combustion engines.

BACKGROUND OF THE INVENTION

Emissions controls for internal combustion engines are becomingincreasingly important in transportation and energy applications. Oneclass of pollutants of concern is oxides of nitrogen (NOx). NOx formduring combustion in internal combustion engines.

One effective NOx treatment system is a lean NOx adsorber (LNA), alsoknown as NOx trap. LNA systems need to be periodically regenerated. Thatis, over time, a reductant is needed to treat NOx traps to permitfurther NOx removal to take place. It is desirable to provide anefficient means of regeneration.

As discussed in, by way of example, PCT/International Publication No. WO00/76637, there are a variety of reductants available for NOx trapregeneration. By way of example, many hydrocarbons, carbon monoxide (CO)and hydrogen can be used as reductants.

Hydrogen is especially effective as a reductant (see U.S. Pat. No.5,953,911). Also, hydrogen is advantageous in regard to the emissionsgenerated when hydrogen is used as a reductant since the products arewater and nitrogen. Other carbon-based reductants such as CO can also beuseful, however, carbon-based reductants result in production of thegreenhouse gas carbon dioxide.

Hydrogen is difficult to store and is generally not readily available.However, hydrocarbons are readily available since internal combustionengines typically use hydrocarbons as fuel. As hydrocarbons comprisehydrogen atoms, they provide a possible source of hydrogen. Ahydrocarbon fuel may be passed through a reformer to yield hydrogen.

Further, while hydrogen is an excellent reductant, any regenerationprocess that takes advantage of hydrogen runs the risk of expellinghydrogen with exhaust gas when regeneration is complete. This isundesirable due to the flammability of hydrogen. Also, regenerationusing hydrogen from a hydrocarbon source consumes a potential fuel.Therefore, improving regeneration efficiency not only reduces expulsionof untreated NOx, it also helps to reduce consumption of hydrocarbonsotherwise available as a fuel.

NOx emissions can also be reduced by managing combustion. NOx emissionscan be reduced by using certain gaseous fuels in place of heavyhydrocarbons.

Examples of such fuels include natural gas, methane and propane. Evenwith gaseous fuel, however, NOx emissions are not insignificant.

Developments in gaseous combustion processes have sought to address NOxemissions problems. Spark ignited gaseous fuel engines, wherein apremixed charge of air and gaseous fuel is ignited with a spark withinthe combustion chamber, have resulted in further reductions of NOx.Also, high pressure directly injected gaseous fuel, ignited by anignition source such as a small quantity of a more readilyauto-ignitable pilot fuel introduced within the engine combustionchamber, yields an improvement over diesel-fuelled engines by reducingthe emissions levels of NOx depending on the gaseous fuel chosen.However some NOx is still generated in such engines and therefore, it isdesirable to reduce this pollutant.

This invention provides an efficient means of regenerating NOxadsorbers.

SUMMARY OF THE INVENTION

The invention is directed to an efficient method and apparatus forregenerating lean NOx adsorbers. A method is disclosed providing abypass strategy for regenerating lean NOx adsorbers efficiently.

A method is disclosed for regenerating a lean NOx adsorber efficientlyby providing an easily recognizable marker indicating the completion ofa regeneration cycle. This allows for real time monitoring ofregeneration or a closed-loop regeneration method. A preferred method ofregenerating a lean NOx adsorber that is used to remove NOx from exhaustgas generated by combustion of a fuel in a combustion chamber of anoperating internal combustion engine comprises:

-   -   (a) determining a target regeneration flow of the exhaust gas        through the lean NOx adsorber,    -   (b) directing a regeneration flow of the exhaust gas through the        lean NOx adsorber, the regeneration flow established by one of        either:        -   bypassing a bypass flow of the exhaust gas around the lean            NOx adsorber when the target regeneration flow is less than            the flow of the exhaust gas from the engine, resulting in            the regeneration flow being substantially the same as the            target regeneration flow, or        -   directing substantially all of the exhaust gas through the            lean NOx adsorber when the target regeneration flow is            greater than the exhaust gas flow from the engine; the flow            of the exhaust gas from the engine and the bypass flow are            determined by reference to at least one of:        -   (1) engine speed,        -   (2) engine load,        -   (3) engine intake manifold temperature,        -   (4) intake air mass flow,        -   (5) engine inlet fuel flow,        -   (6) engine intake manifold pressure,        -   (7) measured engine exhaust gas flow,        -   (8) exhaust gas temperature,        -   (9) exhaust gas pressure;    -   (c) reacting, within the exhaust gas and upstream of the lean        NOx adsorber, the first quantity of the reductant to maintain a        lambda of the regeneration flow of less than one across the lean        NOx adsorber.

The method can be practiced with the reductant being hydrogen. Themethod can also be practiced with the reductant being a hydrocarbon, andin a preferred example, the hydrocarbon is methane. A further aspect ofthe method can comprise reforming a second quantity of the hydrocarbonwithin the exhaust gas upstream of the lean NOx adsorber to introducehydrogen into the regeneration flow.

In a preferred method the fuel that is burned in the engine is the sameas, or is interchangeable with, the reductant. In a further embodiment,the bypass flow is directed through a second lean NOx adsorber.

A method is also provided of operating an internal combustion engineequipped with an aftertreatment system for removing NOx from exhaust gasgenerated by combustion of a fuel in at least one combustion chamber ofthe engine. The method comprises directing all of the exhaust gasthrough a lean NOx adsorber during normal operation of the engine, andperiodically regenerating the lean NOx adsorber during a regenerationcycle defined by a regeneration cycle start time and a regenerationcycle end time. The regeneration cycle includes:

-   -   (a) determining a target regeneration flow of the exhaust gas        through the lean NOx adsorber,    -   (b) directing a regeneration flow of the exhaust gas through the        lean NOx adsorber, the regeneration flow established by one of        either:        -   bypassing a bypass flow of the exhaust gas around the lean            NOx adsorber when the target regeneration flow is less than            the flow of the exhaust gas from the engine, resulting in            the regeneration flow being substantially the same as the            target regeneration flow, and        -   directing substantially all of the exhaust gas through the            lean NOx adsorber when the target regeneration flow is            greater than the flow of the exhaust gas from the engine.            the flow of the exhaust gas from the engine and the bypass            flow are determined by reference to at least one of:        -   (1) engine speed,        -   (2) engine load,        -   (3) engine intake manifold temperature,        -   (4) intake air mass flow,        -   (5) engine inlet fuel flow,        -   (6) engine intake manifold pressure,        -   (7) measured engine exhaust gas flow,        -   (8) exhaust gas temperature,        -   (9) exhaust gas pressure,    -   (c) reacting, within the exhaust gas and upstream of the lean        NOx adsorber, a reductant to maintain a lambda of the        regeneration flow of less than one across the lean NOx adsorber.

In a preferred method the fuel is the same as, or is interchangeablewith, the reductant.

A further aspect of this method comprises introducing hydrogen into theregeneration flow by reforming a second quantity of the hydrocarbonwithin the exhaust gas upstream of the lean NOx adsorber. In a preferredexample, the hydrocarbon is methane. In a further embodiment, hydrogenis introduced into the regeneration flow by reforming the methane withinthe exhaust gas upstream of the lean NOx adsorber.

With regard to the introduction of a hydrocarbon comprising methane intothe aftertreatment system, in one embodiment of the method, thehydrocarbon can be oxidized within the exhaust gas prior to directingthe bypass flow around the lean NOx adsorber. However, in a preferredembodiment the hydrocarbon is oxidized within the regeneration flow. Inthese embodiments, the first quantity of the hydrocarbon can be directedinto the exhaust gas by at least one of a valve or an injector.

In another embodiment of the method of operating an internal combustionengine equipped with an aftertreatment system, the regeneration cycleend time is based on the lambda of the regeneration flow downstream ofthe lean NOx adsorber being representative of an oxygen potential belowa pre-determined threshold concentration. In a further embodiment, theregeneration cycle is based on a concentration of the reductantdownstream of the lean NOx adsorber being above a pre-determinedthreshold concentration.

In another embodiment of the introduction hydrogen into the regenerationflow by reforming a second quantity of the hydrocarbon within theexhaust gas upstream of the lean NOx adsorber, the regeneration cycleend time is based on a concentration of at least one of CO or H₂downstream of the lean NOx adsorber being above a pre-determinedthreshold concentration.

In another embodiment of the method of operating an internal combustionengine equipped with an aftertreatment system, the regeneration flow iscontrolled by at least one valve. In a particular embodiment, theregeneration flow is controlled by a bypass valve in a bypass line andan exhaust valve in an exhaust line. For greater control over theregeneration and bypass flows, each one or both of the bypass valve andthe exhaust valve can be a variable control valve.

In another embodiment of the method of operating an internal combustionengine equipped with an aftertreatment system, the regeneration cyclestart time is determined based on the measurement of a NOx concentrationwithin the exhaust gas downstream of the lean NOx adsorber, with thestart time occurring when the measured NOx concentration is higher thana threshold concentration, which is determined by reference to a NOxconcentration of the exhaust gas exiting from the engine.

In embodiments of the method that employ methane as the hydrocarbon, themethod can further comprise burning fuel in the combustion chamber togenerate the exhaust gas wherein the lambda of the exhaust gas is lessthan one when operating in a predefined low load, low speed mode.

In a preferred example the engine is a direct injection engine.

In another embodiment of the method of operating an internal combustionengine equipped with an aftertreatment system, exhaust gas is directeddownstream of the lean NOx adsorber through a clean-up catalyst duringthe regeneration cycle. The clean up catalyst can remove NOx orreductant. In a preferred example, the clean up catalyst removeshydrogen sulfide from the exhaust gas.

The method may further comprise directing the exhaust gas through aparticulate filter upstream of the lean NOx, adsorber, or in anotherexample, directing bypass flow through a second lean NOx adsorber.

A further method is disclosed for operating an internal combustionengine equipped with an aftertreatment system for removing NOx fromexhaust gas generated by combustion of a fuel in at least one combustionchamber of the engine. The method comprises:

-   -   (a) directing all of the exhaust gas through a lean NOx adsorber        during normal operation of the engine;\    -   (b) periodically regenerating the lean NOx adsorber using a        predetermined regeneration strategy selected from one of a high        load strategy, a midrange load strategy and a low load strategy,        the regeneration strategy causing the exhaust gas pass through        the lean NOx adsorber:        -   during the high load strategy: reacting, upstream of the            lean NOx adsorber, a reductant to maintain a lambda of the            exhaust gas of less than one across the NOx adsorber,        -   during the low load strategy: burning the fuel in the            combustion chamber generating the exhaust gas wherein the            lambda of the exhaust gas is less than one, and        -   during the midrange load strategy, causing the lambda of the            exhaust gas to be less than one across the NOx adsorber by:            burning the fuel with the combustion chamber in a rich            environment, and reacting a reductant upstream of the lean            NOx adsorber.

An aftertreatment system is provided for removing NOx found in exhaustgas produced during combustion of a fuel within a combustion chamber ofan operating internal combustion engine. This system comprises:

-   -   (a) an exhaust line for directing the exhaust gas from the        engine,    -   (b) a lean NOx adsorber disposed in the exhaust line for        removing NOx,    -   (c) a regeneration catalyst disposed in the exhaust line        upstream of the lean NOx adsorber, with such regeneration        catalyst capable of oxidizing a reductant,    -   (d) a reductant line for delivering the reductant from a        reductant store to the exhaust line upstream of the regeneration        catalyst,    -   (e) a reductant flow control disposed in the reductant line for        controlling reductant flow into the exhaust line    -   (f) a bypass line for directing the exhaust gas around the lean        NOx, adsorber,    -   (g) at least one bypass flow control capable of controlling flow        of the exhaust gas through the bypass line,    -   (h) a controller, and    -   (i) at least one sensor providing control information to the        controller, the controller capable of adjusting the at least one        valve in response to the control information.

With this aftertreatment system, the reductant can be hydrogen and in apreferred example, the reductant is a gaseous hydrocarbon. In thisembodiment the regeneration catalyst is capable of reducing the gaseoushydrocarbon to provide hydrogen with the exhaust gas.

In a preferred embodiment of the aftertreatment system, the regenerationcatalyst comprises a reformer in series with an oxidation catalyst. Theregeneration catalyst can also be an oxidation catalyst. In anotherembodiment, the regeneration catalyst comprises an oxidation catalystcombined with a reformer.

The aftertreatment system can further comprise a second close-coupledcatalyst proximate to the engine for oxidizing the reductant when theexhaust gas proximate to the regeneration catalyst is at a temperaturebelow a predetermined threshold temperature. The predetermined thresholdtemperature is determined as the temperature below which the reductioncatalyst is unable to efficiently oxidize the reductant.

The aftertreatment system can further comprise an injector for injectingthe reductant into the exhaust line.

The bypass valve can be a variable control valve for improved modulationof flow through the by-pass line. In another embodiment the reductantstore can be a fuel system of the engine.

A further embodiment introduces a second quantity of the reductant suchas hydrogen or a hydrocarbon through a second gas line to deliver it tothe oxidation catalyst.

The system may further comprise a particulate filter disposed in theexhaust line downstream of and proximate to the regeneration catalyst ora second lean NOx adsorber in the bypass line.

A second regeneration catalyst may also be disposed in the bypass lineupstream of the second lean NOx adsorber.

The aftertreatment system can further comprise a clean-up catalystdisposed downstream of the lean NOx adsorber. The clean-up catalyst iscapable of removing NOx or reductant, and in a preferred example,hydrogen sulfide from the exhaust gas.

Further aspects of the invention and features of specific embodiments ofthe invention are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

In drawings which illustrate non-limiting embodiments of the invention:

FIG. 1 shows a schematic of a NOx management system according to oneembodiment of the invention.

FIG. 2 shows a graphical representation of properties of the exhaust gasplotted against time. Included are some system properties over aregeneration cycle.

FIG. 3 shows an engine-operating map of torque against speed with flowgradients over a regeneration cycle provided for use in a closed-loopcontrol strategy.

FIG. 4 shows a graph of flow versus temperature of the exhaust gas outof the engine and through the NOx catalyst during regeneration.

FIG. 5 shows an alternate embodiment of the subject invention with atwo-bed by-pass aftertreatment system.

FIG. 6 shows an alternate embodiment of a control strategy for thesubject invention representing an engine operating map of torque againstspeed with flow gradients over a regeneration cycle provided for use ina closed-loop control strategy utilizing both in-line and in-cylinderregeneration.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

A method of regenerating a NOx adsorber that is used to treat exhaustgases created during combustion in the combustion chamber is disclosed.A hydrocarbon, preferably methane, is introduced into the exhaust linewherein the hydrocarbon is oxidized and reformed within the exhaust lineto generate hydrogen, which is used to regenerate the NOx absorber. CO,as well as hydrogen, is generated during reformation of methaneresulting in a regeneration mixture that includes both hydrogen and CO.The aftertreatment system is capable of directing an amount of exhaustgas to by-pass the NOx adsorber during regeneration for the purposes ofreducing regeneration flow, hydrocarbon consumption, emissions, andregeneration time. Specific markers, indicative of the properties of theexhaust gas, can be used to identify completion of regeneration.

FIG. 1 is a schematic showing an aftertreatment system according to apreferred embodiment of the invention. An exhaust line 22 carriesexhaust gases flowing in the direction of arrow 20 from an engine block11 to an outlet in the direction of arrow 31. Components of a NOxaftertreatment system can be disposed in exhaust line 22 such thatexhaust gases are carried to NOx adsorber 46 as indicated by arrow 56.Regeneration catalyst 42 is disposed in exhaust line 22 upstream of NOxadsorber 46.

By-pass line 12 is capable of carrying a portion of the exhaust gasesaround adsorber 46 as may be desirable while absorber 46 is beingregenerated. The exhaust gases can be directed through by-pass line 12as indicated by arrow 18 by opening by-pass valve 14. By-pass valve 14can be disposed anywhere along by-pass line 12. In the embodiment shown,by-pass line 12 branches off from exhaust line 22 at a junction 16 andrejoins exhaust line 22 at a junction 48 downstream from NOx adsorber46.

Valves 13 and 14 are provided to help control the flow of exhaust gasesthrough line 22 and by-pass line 12 during regeneration.

Although not preferred if operating of the subject method and apparatusis possible without it (see discussion below), FIG. 1 also shows a closecoupled catalyst 74 in line 22 physically proximate to engine block 11.A hydrocarbon, preferably methane gas, can be introduced just prior tocatalyst 42 and/or catalyst 74.

Hydrocarbon valves 28 and 29 are disposed in respective main line 26 andclose couple line 27, each of which branches off from store line 34.Store line 34 is connected to store 36 from which methane is allowed toflow as indicated by arrow 50. Flow direction 51 and 52 along lines 26and 27 are also provided.

Lambda sensor 71 is used to measure lambda. Lambda, is defined herein asa measure of the oxygen potential of the exhaust gas. A lambda sensormeasures this potential. Generally, a lambda value above 1 denotes ahigh oxygen potential and a lambda value below 1 denotes a low oxygenpotential. A rich exhaust gas environment is an environment with alambda value below 1 while a lean exhaust gas environment is anenvironment with a lambda value above 1. Lambda sensor 71 measureslambda in the exhaust gas after adsorber 46 and also near engine block11 as shown by the intersection point of feed lines 61 and 63 withexhaust line 22. NOx sensor 72 is used to measure NOx levels afteradsorber 46 and near engine block 11 as shown by the intersection pointof feed lines 62 and 64 with exhaust line 22.

Temperature sensor 73 is also used to measure temperatures before andafter catalyst 42 as show by the intersection point of feed lines 65 and66 with exhaust line 22.

Finally, each of sensors 71, 72 and 73 feed information to controller 70through respective feed lines 67, 68 and 69. Line 60 provides enginedata to controller 70.

Controller 70 drives valves 13 and 14, through feed lines 75 and 76, andvalves 28 and 29, through feed lines 77 and 78.

FIG. 2 provides a graph demonstrating a sample set of conditions withinthe exhaust gas at a typical mid-range engine speed and load plottedagainst time. Line 500 is lambda of the exhaust gas measured at point C(refer to FIG. 1). Line 502 is the temperature of the exhaust gasmeasured in degrees Celsius at point A (refer to FIG. 1). Line 504 isthe flow of the exhaust gas at point B measured in kg/hr. Line 506 isthe NOx concentration of the exhaust gas measured downstream of adsorber46 at point D (refer to FIG. 1), in ppm. Line 508 is lambda of theexhaust gas at point B (refer to FIG. 1) and line 508 overlaps line 500except for the dashed line indicated by reference number 508 between thestart and end of the regeneration cycle. Line 510 represents a lambdavalue of 1, above which the exhaust gas has high oxygen potential andbelow which the exhaust gas has a low oxygen potential. Line S providesthe approximate start time of a regeneration cycle. Line O provides theearliest end to a regeneration cycle. Line F provides the end of theregeneration cycle in the example shown.

FIG. 3 provides an engine map of torque versus speed. Lines 900 through906 provide gradient lines that demonstrate the boundaries at which100%, 50%, 35% and 20% of the total exhaust gas flow is directed throughthe NOx adsorber during regeneration. Line 908 defines the boundary ofthe engine-operating map.

FIG. 4 provides a graph of flow or space velocity of the exhaust gasplotted against temperature. Line 800 provides an example of exhaust gasproperties out of engine block 11 over all operating conditions of theengine. Line 802 provides target properties of the exhaust gas throughNOx adsorber 46 during regeneration.

Referring to FIG. 5, an alternate embodiment of the subject invention isprovided wherein by-pass system 968 is a two bed system. Exhaust line972 directs exhaust gas in direction 974 away from engine block 970.Reformers 976, 978 are disposed in lines 977, 979 wherein exhaust gascan be directed according to arrows 984 or 986. Adsorbers 980 and 982are also disposed in lines 977, 979. The resulting exhaust gas isdirected from system 968 in direction 988. Valves 990, 992 are disposedin each of lines 977, 979. Valves 994, 996 are also provided in storelines 995, 997 to control flow of gas from store 993 in direction 998.

FIG. 6 provides an alternate torque versus speed map for controlling theoperation of the subject aftertreatment system. Line 950 definestransition region 952 for regeneration wherein a combination ofin-cylinder and in-line regeneration is done.

Lines 954, 956, 958 and 960 provide the same gradient lines, as found inFIG. 3, that demonstrate the boundaries at which 100%, 50%, 35% and 20%of the total exhaust gas flow is directed through the NOx adsorberduring regeneration. Line 962 defines the boundary of theengine-operating map.

In the NOx aftertreatment systems of FIG. 1, exhaust gas is generated bycombustion events within one or more combustion chambers disposedupstream of engine exhaust line 22 in engine block 11. Exhaust gasresults from the combustion of fuel (lean burn combustion when a NOxadsorber is used for aftertreatment purposes) such as natural gas or amixed fuel that includes natural gas or methane. The fuel is, ingeneral, either directly injected into the combustion chamber orpre-mixed with a quantity of air to create a fumigated charge or is acombination of the two wherein a premixed charge and directly injectedcharge drive the engine. In each case, spark ignition, hot surfaceignition or compression ignition are utilized to initiate the combustionprocess within the combustion chamber.

During normal operation of the engine valve 14 is closed and exhaust gasflows along exhaust line 22. The exhaust gas also passes through NOxadsorber 46, which removes NOx. By way of example, during normaloperation, NOx adsorber is under lean operating conditions, that is,with an excess of oxygen available in the exhaust gas, NOx is driven to(NO₃)₂ by way of the following reactions:NO+½O₂(Pt)→NO₂  (1)XO+2NO₂+½O₂→X(NO₃)₂  (2)where X is a washcoat, as is well known to those skilled in thistechnology.

Eventually NOx adsorber 46 will become less effective at removing NOx asX(NO₃)₂ uses up adsorbing sites in adsorber 46. NOx slip is used toexpress a percentage increase in NOx emissions above a baseconcentration of NOx. NOx slip can be used to determine when anunacceptable level of NOx is being expelled. When this unacceptablelevel is reached, adsorber 46 is regenerated. Upon regeneration the NOxadsorber returns to removing NOx from the exhaust gas. Controller 70determines when NOx adsorber 46 needs regenerating. This can be donethrough an open loop control, based on selected parameters from theengine map, or closed loop control, based, in part, on direct readingsof the NOx concentration within the treated exhaust gas. By way ofexample, one such open loop control uses a calibration of theaftertreatment system over a range of engine operating conditions toestimate the time at which adsorber 46 needs regeneration. That is, thecontroller monitors such variables as the engine load and speed,determining from a look-up table, the time for regeneration. With thismethod, the system is calibrated such that the engine operatingconditions, which are indicative of NOx production, are used to estimatewhen regeneration for the NOx adsorber is desirable. Conditions such astorque, speed, intake air mass flow, the fuel flow into the engine,intake manifold temperature, intake manifold pressure, as well asothers, can be used for open loop control.

A closed loop control for determining the commencement of a regenerationcycle could also be used. By way of example, one such control monitorsNOx levels within exhaust line 22 downstream of adsorber 46 with sensor72 through line 64 and near the engine through line 62. Controller 70can commence regeneration once the ratio of NOx at point C to NOx out ofblock 11 exceeds a predetermined threshold NOx slip level.

During the regeneration cycle, controller 70 needs to provide H₂ and/orCO to NOx adsorber 46 and do so in a rich exhaust gas environment(oxygen depleted environment). The controller can control exhaust gasflow and the introduction of methane to provide a regeneration strategythat will help reduce hydrocarbons used for regeneration, and reduce thetime required for regeneration. One hydrocarbon that can be used ismethane.

During regeneration, the following provides a set of reactions foundacross catalyst 42:CH₄+2O₂→CO₂+2H₂O  (3)CH₄+½O₂→CO+2H₂  (4)CH₄+H₂O→CO+3H₂  (5)CO+H₂O⇄CO₂+H₂  (6)where reaction (6) can be held in equilibrium depending on exhaust gastemperature. Note also, that equation (3) may occur but is notpreferred. The CO and H₂ generated according to equations (4) through(6) are then used for regeneration as follows:X(NO₃)₂→XO+2NO+ 3/2O₂  (7)X(NO₃)₂→XO+2NO₂+½O₂  (8)NO+H₂→H₂O+½N₂  (9)NO₂+2H₂→½N₂+2H₂O  (10)NO+CO(Rh)→½N₂+CO₂  (11)NO₂+2CO→½N₂+2CO₂  (12)where X is provided in a washcoat. A lambda less than 1, which denotes alow oxygen potential in the exhaust gas, favors reactions (7) through(12); this is not the case, in general, when lambda is above 1.

Controller 70 determines a regeneration strategy based, generally:

-   -   on the exhaust gas flow,    -   the exhaust gas temperature,    -   desired exhaust gas flow chosen considering the reactive        capacity of catalyst 42 at a given exhaust gas temperature,    -   lambda of the exhaust gas from the engine, adsorber 46 and/or        catalyst 42 throughout a regeneration cycle, and    -   the type of adsorber 46 and catalyst 42.

The catalyst 42 is chosen to suit the engine used and the operatingconditions contemplated for the engine. The regeneration strategy for agiven regeneration cycle can be done by an open loop or closed loopstrategy. The regeneration strategy can control the quantity and rate ofintroducing the methane into the aftertreatment system from store 36 andthe quantity of by-pass flow by controlling valves 13 and 14. The goalduring regeneration is to efficiently provide an exhaust gas environmentwherein lambda is below one, thereby promoting reactions (7) through(12). This is realized when reductants are provided to the NOx adsorberthat remove oxygen released according to the reactions above. However,oxidation through the NOx adsorber with other reductants such as methaneor other hydrocarbons can also provide the necessary rich exhaust gasenvironment. It is also desirable to provide conditions where allhydrocarbons are used efficiently to reduce NOx, and minimize the timerequired for regeneration.

In an open loop strategy, the controller is preferably calibrated todirect flow of exhaust gas through the adsorber and methane into theexhaust gas based on the engine speed and load just prior to and duringregeneration. Engine intake manifold temperature, intake air mass flow,the fuel flow into the engine or intake manifold pressure can also beused as indicators for controlling regeneration. A constant regenerationcycle time can also be used in certain static operating conditions whenload and speed remain relatively constant over extended periods of time.Such an open loop strategy employs an engine calibration that considersone or more engine operating conditions, each of which is indicative ofat least one of exhaust gas temperature, flow and lambda value. Thecontroller is calibrated to direct a desired flow of exhaust gas throughthe NOx adsorber based on the characteristics of catalyst 42 andadsorber 46.

The flow through NOx adsorber 46 during regeneration is referred toherein as the regeneration flow. A look-up table is used to determinewhether the exhaust gas flow exceeds the desired regeneration flow and,if so, directs excess exhaust gas around adsorber 46 via by-pass line12. This is referred to as the by-pass flow if exhaust gas is by-passedduring regeneration. The desired by-pass flow is achieved by adjustingvalves 13 and 14 to match the target regeneration flow though adsorber46.

Referring to the engine map of FIG. 3, the percentage flow of exhaustgas through NOx adsorber 46 is provided based on the torque and speed ofthe engine and this percentage is reduced as speed and torque increases,as is seen in contour lines 900 through 906. Operating conditionsfalling above or to the right of line 900 show a reduction in theproportion of the total exhaust gas that flows through adsorber 46.Below and to the left of line 900, controller 70 does not open valve 14allowing all exhaust gas to flow through adsorber 46.

The look-up table for this open loop control also provides a targetmethane concentration upstream of catalyst 42. The engine operatingconditions provide information about the exhaust gas temperature andlambda of the exhaust gas from block 11. The lambda of the exhaust gas,determined based on the engine operating conditions, and the flow of theexhaust gas determines in part the amount of methane required togenerate a sufficiently rich exhaust gas environment to supportefficient regeneration.

A closed loop strategy could also be used. In a closed loop systemlambda may be measured out of engine block 11 by sensor 71 and thetemperature may be measured prior to catalyst 42 by sensor 73 throughline 66 and after catalyst 42 through line 65. The load and speed of theengine may be used by the controller to infer the exhaust gas flow basedon look-up tables or a flow meter within the exhaust line may also beused for complete closed loop control. The look-up table along withsensor information are used to determine the flow of methane to beintroduced into exhaust line 22 and how much flow of exhaust gas, ifany, to direct through valve 14 and line 12 during regeneration. Whenexhaust gas flow is too high for catalyst 42 to allow complete oxidationand reformation of methane or too high to regenerate catalyst 46efficiently, some flow is directed into by-pass line 12 until thedesired flow is met.

If temperature prior to and after catalyst 42 is too high or too low,the methane quantity can be increased or reduced according to thosetemperature readings. For example, if the temperature falls below apredetermined temperature set during calibration and based, in part, onthe catalyst chosen, methane could be reduced to ensure that the exhaustgas temperature is elevated to an acceptable level to support thereformation reaction (5) set out above (assuming the inlet mixture tocatalyst 42 originally had excess fuel). Further, if the post-catalysttemperature is too high, the methane quantity can be shut-off to avoidoverheating the catalyst and damaging it during regeneration. Such astrategy can employ a series of cycles whereby the methane flow throughvalve 28 is opened and closed a few times through one regeneration cycleto ensure that adsorber 46 is regenerated while protecting catalyst 42.

Likewise, lambda sensor 71 can allow the controller to adjust thequantity of methane introduced through valve 28 to ensure that theexhaust gas was rich enough to approach target regeneration efficiencyacross adsorber 46 according to reactions (7) through (12) set outabove.

Also, a lambda sensor could be provided after catalyst 42 and beforeadsorber 46 rather than, or in addition to, sensor 71 provided. Thiswould monitor the oxygen potential out of catalyst 42 to provide forefficient regeneration through adsorber 46. That is, if the flow ofmethane through line 26 is unknown, then the lambda sensor could be usedto close loop control the flow of methane to help provide for a targetlambda in the regeneration flow prior to regeneration of NO, adsorber46.

An optional close-coupled catalyst 74 is also available to increaseexhaust gas temperatures when desired. The proximity of catalyst 74 toblock 11, helps ensure that exhaust gas is not too cool to oxidizemethane within the exhaust gas environment. Therefore, when thecontroller detects an exhaust gas temperature below a threshold amount,valve 29 will provide methane upstream of catalyst 74, heating andoxidizing the exhaust gas well upstream of adsorber 46. Catalyst 74 canalso be used to produce CO and hydrogen for use in regeneration as wasdone with catalyst 42.

As noted above, these closed loop strategies are preferred but they arenot necessary. The open loop strategy discussed above utilizing acalibration of the system that provides a target methane injection rateand quantity over a regeneration cycle that is based on the engineoperating parameters such as load and speed, could eliminate dynamicmonitoring and the added complexity in hardware and software for thesystem. However, the trade-off is that such a strategy is more likely toregenerate incompletely or to regenerate with a higher methane penalty.

The controller can determine completion of a regeneration cycle byreference to a closed or open loop control. In a closed loop control,the controller can use readings from lambda sensor 71 downstream ofadsorber 46 to determine when the oxygen potential within line 22downstream of adsorber 46 is decreasing. Referring to reactions (7)through (12), once most nitrogen has been released from adsorber 46,oxygen potential begins to decrease as oxygen is no longer beingreleased from adsorber 46. Other sensors may be appropriate for closedloop monitoring of regeneration cycle completion, including a CO or H₂sensor that detects increases in CO or H₂ downstream of adsorber 46.These increases would occur when oxygen is no longer released from theadsorber causing H₂ and CO to pass through the adsorber unreacted.

An open loop control could also be used relying on the calibration ofthe system wherein regeneration time is pre-determined based on engineoperating conditions such as speed, load, intake air mass flow, the fuelflow into the engine, intake manifold temperature and pressure.

Referring again to FIG. 3, as either or both speed and load of theengine at the commencement of and during a regeneration cycle increase,controller 70 commands valve 14 to open when load and speed fall aboveline 900. Valves 13 and 14 can be variable control valves providing fora wide range of operating conditions as the engine operating parameterscontinue to generate exhaust gas flows above a target flow throughcatalyst 42 or adsorber 46 determined, in part, by the properties ofcatalyst 42. Therefore, when controller 70 determines a desired exhaustgas flow, valves 13 and 14 can be adjusted to maintain thispre-determined flow of exhaust gas through line 22 during regeneration.

To simplify the system, an alternative to variable flow control valvesused for valve 13 and 14 are two position valves (or, for that matter,other multiple position valves). Here, the controller can elect from oneof three possible settings. Valve 13 can be fully open or partiallyopen. Valve 14 can be closed or fully open. Therefore, controller 70 canselect a position for each valve according to the engine operatingparameters in order to match exhaust flow through line 22 to apre-determined target value. That is, at low speed and load, valve 13 isopen fully and valve 14 is closed. At higher loads and speeds, valve 13is fully opened and valve 14 is fully opened. At still higher speeds andloads, valve 13 is partially closed and valve 14 is opened.

Other valve configurations can be used as well. More flexibility for thecontroller to manage flow through line 22 during regeneration helps thecontroller to meet a target pre-determined flow rate for each operatingcondition. One trade-off is that such flexibility may result in a moreexpensive system that requires more expensive valves and morecomplicated software to control those valves.

As would be understood by a person skilled in the technology, valves 13and 14 can be any flow control mechanism and need not be limited tovalves.

Referring to FIG. 4, flow and temperature over the range of engineoperating conditions are provided. Area 800 shows typical exhaust gasflow and temperature conditions expelled from block 11. Area 802provides the controller a desired operating range for temperature andflow of exhaust gas through regeneration catalyst 42 duringregeneration—which provide the exhaust gas conditions which then allowthe conditions necessary for regeneration of NOx adsorber 46. Therefore,when the flow out of block 11 is above area 802, flow through bypassline 12 can be used to bring the exhaust flow through adsorber 46 towithin area 802 and below the upper limit flow or upper flow of therange. Ideally, regeneration flow is targeted to a desired flow withinthis range, however, depending on such things as exhaust gas flow, valvereaction times in the system, pressure and temperature changes, adifferent regeneration flow within the range defined by 802 is all thatcan be maintained. When the temperature falls below area 802 (to theleft of area 802) at catalyst 42, additional heat can be generatedthrough the operation of the engine as described below or using closecoupled catalyst 74, proximate to block 11, as described above andbelow.

Referring to FIG. 2, selected properties of the system are plotted overthe course of a partial adsorbing cycle and an entire regenerationcycle. Referring to FIG. 1, lambda at points B and C (lines 508 and 500,respectively), temperature and space velocity at point A (lines 502 and504, respectively) and NOx at point D (line 506) are all shown, plottedagainst time. The example provided is representative of operation of theaftertreatment system when an engine is running at a typical midrangespeed and load.

Referring to line 506, NOx concentrations increase gradually until thecontroller determines that the level has exceeded a pre-determinedthreshold—this could be done by monitoring the engine operatingparameters or measuring the NOx concentration. Regeneration thencommences with opening of valve 14. Commencement of regeneration isshown at time S. Opening valve 14 drops the space velocity or flowthrough line 22, line 504, at time S. Methane is also directed into line22 causing lambda to drop to a level below 1 between catalyst 42 andadsorber 46 (line 508). Lambda following the NOx adsorber also dropsafter regeneration is complete (line 500), but during regeneration ofthe adsorber, it is maintained near a lambda value of 1 as the richmixture entering the adsorber releases oxygen from the oxides ofnitrogen resulting in a leaner mixture expelled from adsorber 46 thanthat entering adsorber 46. Eventually, however, no further oxygen isreleased from adsorber 46 and lambda falls until the lambda out ofadsorber 46 is the same as lambda into adsorber 46 (line 508). Once athreshold lambda out of adsorber 46 is detected, valve 14 is closedalong with valve 28. Immediately, the flow begins to rise, line 504, asall exhaust gas is again routed through exhaust line 22. Soon, lambdabegins to rise resulting in a lean exhaust gas environment, lines 500and 508.

Note, that the regeneration cycle is complete at time F in FIG. 2. Thisis, in practice, a delayed end of the regeneration cycle. Preferably,the regeneration cycle would be completed sometime between time O andtime F when lambda after the NOx adsorber (line 500) drops below apre-determined threshold amount and before it matches lambda upstream ofthe NOx adsorber (line 508).

During the regeneration cycle, the NOx levels out of line 22 increasesubstantially, as the engine is continuing to operate without NOxtreatment of the exhaust gas routed through by-pass line 12, line 504.Once regeneration is complete, however, NOx quickly falls as all exhaustgas is routed through recently regenerated adsorber 46. Therefore, aswell as limiting fuel consumption (consumption of methane), shortregeneration times also limit the amount of NOx emitted duringregeneration through by-pass line 12. The longer the period of timeneeded for regeneration, the more cumulative exhaust gas flows throughby-pass line 12. The target cycle is based on generating as muchreductant per unit methane injected over the shortest time period. Thisis a function of variables such as the temperature of the exhaust gas,flow of exhaust gas, catalyst specifications, and lambda of exhaust gas,since a higher lambda requires more methane to burn off the oxygenpresent but more oxygen is available to generate CO. Preferably,regeneration cycles should be kept to less than 5% of operating time ofthe engine. Also, as noted above, a greater flow of exhaust gas routedthrough by-pass line 12, results in higher NOx emissions since by-passline 12 does not generally include a separate NOx adsorber.

FIG. 5, shows an alternative embodiment of the aftertreatment system.Here a second NOx adsorber and catalyst is disposed in the by-pass lineto treat NOx through that line during regeneration. That is either oneof line 977 or line 979 act as the bypass lines during regeneration ofthe either one of adsorber 980 or adsorber 982. Valves 994, 996, 990 and992 all work to control which of the adsorber is being regenerated. Forexample, for regeneration of adsorber 980, bypass line is line 979.Here, a regeneration cycle is begun when exhaust gas is directedaccording to a control strategy (see FIG. 3), through reformer 976.Excess exhaust gas is bypassed through adsorber 982 during regenerationin the same manner as exhaust gas was also bypassed through line 12referring to FIG. 1. Here, however, the excess exhaust gas is treated byNOx adsorber 982.

In the example, the reductant source, methane, is directed from store993 through valve 994 to line 977. At the same time, valves 990 and 992are opened according to the desired split of exhaust gas through eachvalve for the purposes of regeneration. Eventually, NOx adsorber 982, aswell, would need to be regenerated. In which case valve 994 would closeand 996 would open and the acting bypass line would be 977.

This system in general could also be extended to a multi-bed system with3 or 4 or 5 or more beds with one “off-line” at any one time. Thebenefit here is improved NOx conversion for the same catalyst volume.

Note also, for multi-bed systems—2 or more beds—there is no need for twophysically distinct adsorbers. Parts of a single physical adsorber couldact as an isolated adsorber as well.

Also, while two reformers are shown in FIG. 5, one could be used. Infact, one reformer could be used for other multiple bed designs. Herethe reformer would include lines that routed exhaust around the reformerto each adsorber as well as lines that routed exhaust gas from thereformer through to each adsorber. Valves would control the design ofsuch a system.

In general, while these multi-bed systems provide better conversion,they also tend to add cost and complexity to the system both in terms ofthe architecture and in terms of the control mechanisms.

An alternative method of operating the aftertreatment system that canhelp to reduce regeneration time employs an additional exhaust line thatroutes exhaust gas around catalyst 42 and through adsorber 46 duringregular operation. A valve disposed in this additional exhaust linecould be used such that valve 13, closed during regular operation, wouldbe opened just prior to commencement of regeneration, while maintainingthe catalyst bypass open. This would allow a flow of exhaust gas throughline 22, lighting off catalyst 42 and warming the line prior to aregeneration cycle. When a valve used to bypass catalyst 42 is closed atthe beginning of a regeneration cycle, there can be less time needed toheat line 22 and less time before regeneration can commence.Alternatively, in such an embodiment with an additional exhaust linearound catalyst 42, the flow rate within reformer line 22 can be set toensure a certain amount of exhaust gas is always flowing through line 22eliminating the need for valve 13 by employing a valve to regulate flowthrough catalyst 42 by controlling flow through the additional exhaustline.

Catalyst 42 is generically describe as a bed that promotes reactions (3)through (5) to provide a desired exhaust gas with elevatedconcentrations of H₂ and/or CO and minimal amounts of oxygen. To varyingextents, reactions (3) through (6), a combination of exothermic andendothermic reactions, drive the process across this catalyst. Thiscatalyst can be a reformer that oxidizes methane and promotes reaction(5) to provide H₂ and CO. It can also be a partial oxidation catalyst,which partially oxidizes methane and reforms methane to provide H₂ andCO, see reaction (4). Catalyst 42 can also be a back-to-back oxidationcatalyst and reformer sharing a common boundary surface. This catalystwould first oxidize methane until little oxygen remains within theexhaust gas and then, use excess methane to generate H₂ and CO withinthe reformer. These two catalysts, the oxidation catalyst and reformer,can also be disposed in line 22 in series and need not share a commonboundary surface. Also, a combination reformer and oxidation catalystcould be used that integrates the reformer and oxidation catalysttogether in a mixed catalyst. Each option has balancing cost andefficiency considerations that weigh in any decision as to whichcatalyst to use depending on the aftertreatment system sought.

As noted briefly above, referring again to FIG. 1, an additionalcatalyst, close coupled catalyst 74, is shown positioned near engineblock 11. Some systems need such a catalyst disposed close to the engineto ensure that the exhaust gas is hot enough to support oxidation ofmethane. That is, there are some aftertreatment system designs thatwould benefit from employing a close coupled catalyst near the engineblock so that the exhaust gas temperature under low load and/or speed oridle conditions can be prevented from falling below a threshold limit atwhich stable oxidation of methane in catalyst 42 would be compromised.Therefore, under such conditions, there are advantages in havingclose-coupled catalyst 74 near engine block 11 with line 27 feedingmethane upstream of such catalyst. This catalyst would then eitheroxidize the methane provided from store 36 to heat the exhaust gas to atemperature suitable to allow catalyst 42 to light off satisfactorily.Alternatively, catalyst 74 can provide the rich exhaust gas environmentalong with H₂ and CO needed to regenerate adsorber 46. It would bedesirable here, however, to operate this way only when valve 14 isclosed in order to prevent CO and H₂ from escaping through the by-passline, since this would be inefficient.

An additional method of operating the regeneration cycle under low loadconditions is to burn a fuel rich combustible mixture, preferablycomprising methane, in the combustion chamber within engine block 11.Alternatively, a method wherein fuel burned lean with an injection offuel late in the cycle can be used. Fuel can oxidize in the combustionchamber, or in exhaust or over the catalyst. In each case, this willgenerate an excess of CO and some H₂ while creating a rich exhaust gasenvironment. With this method, no methane needs to be provided tocatalyst 42 when the necessary reductants are present within a richexhaust gas environment. Preferably, such a strategy would be limited toconditions when flow and temperature are low which is typicallyassociated with light load and low speed conditions or idle conditionswhen full flow through adsorber 46 is desirable.

Use of in-cylinder techniques to generate the necessary conditions andreductants to regenerate the NOx adsorber can have drawbacks.

For example, a spark-ignited engine running under lean conditions has anengine out NOx that is relatively low when compared to that resultingfrom operation under stoichiometric conditions. In general, the peak inNOx production occurs at conditions slightly lean of stoichiometricwhere engine out NOx can be an order of magnitude larger than thatproduced near the lean limit. Thus, transitioning from lean to richoperation to create the conditions conducive to NOx adsorberregeneration also results in a substantial increase in engine out NOxemissions. While operating under rich conditions, the NOx adsorberreduces both the engine out and adsorber released NOx to nitrogen.However, when transitioning back to lean operation, the substantialamounts of NO, created during operation just lean of stoichiometric arecaptured by the NOx adsorber.

Under light load/low speed conditions, for example, idle, the mass flowrate of NOx is relatively low and the storage capacity of the NOxadsorber is relatively high. Adsorption phases in excess of 1000 secondscan be realized. Under this condition, the influence of the NOx adsorbedduring the transition from rich to lean operation has relatively littleimpact. For example, if the transition from lean to rich takes fiveseconds to accomplish, and the NOx produced is an order of magnitudelarger than that of lean operation, the adsorption phase would bereduced by 5% to 950 seconds in the example provided. However, as themass flow rate of the engine out NOx increases with speed and load, therelative impact of the additional NOx produced during the transitionstarts to have an impact on the system operation. For example, at anengine operating condition at a higher speed and load relative to idle,the engine out NOx flow rate doubles. Under these conditions, the filltime of the NOx adsorber, based on lean engine out NOx emissions andtemperatures, is reduced to 500 seconds. When using in-cylinderregeneration, the 5-second transition now reduces the fill time by 10%to 450 seconds. Continuing along this line, at some point, thein-cylinder technique is not an effective means to regenerate the NOxadsorber.

Similar results would be expected for a fumigated or port-injected leanburn engine. However, the port-injected lean burn engine is expected tobe more tolerant to in-cylinder regeneration because the transition timefrom lean to rich is expected to be shorter.

Retarding the spark timing would help the situation, possibly extendingthe region where the in-cylinder regeneration could be used. Similarly,if increasing EGR rates were available to reduce the in-cylinder oxygenconcentration, the concern considered above would still exist, but beameliorated.

This issue is not expected to arise for engines using the directinjection of gaseous fuels. For direct injection engines, the use of aninjector provides more flexibility in transition to rich operation. Forexample a combination of increasing the EGR rates, retarding of the gasinjection timing, multiple injections and throttling of the engine isavailable. These conditions would not lead to significant increases inthe engine out NOx levels. Rather, engine out NOx levels would decrease(with a potential increase in particulate matter emissions).

Where in-cylinder regeneration is not effective, one can resort to theuse of the in-line regeneration technique. However, under full flowconditions at relatively low exhaust gas temperatures the in-lineregeneration method may not be efficient. The fuel penalty andhydrocarbon slip can be significantly higher than that associated withthe in-cylinder regeneration. As mentioned above, the in-lineregeneration efficiency can be improved with the use of a close-couplecatalyst, but may still not be satisfactory.

Therefore, the system needs special management consideration when thespeed load region where efficient in-cylinder regeneration is possibledoes not overlap with the region where efficient in-line regenerationcan be realized.

Therefore, referring to FIG. 6, the region where effective in-cylinderregeneration can be realized is below line 950 in FIG. 6. The regionwhere effective in-line regeneration can be realized is above line 950.The region where neither in-cylinder nor in-line regeneration isefficient is defined as area 952 bounded by line 950 in FIG. 6. Withinarea 952, the use of a combination of in-cylinder and in-lineregeneration allows this region of the engine map to be managed. Oneembodiment of the combined method provides for the engine an enrichmentof the combustion environment during regeneration to reduce the oxygenconcentration in the exhaust. At the same time, the spark timing may ormay not be retarded to increase exhaust gas temperature (and reduceengine out NOx emissions). A quantity of reductant is injected upstreamof the regeneration catalyst to react with the species in the exhaustgas to reduce the oxygen potential further and create conditionsconducive to regenerating the NOx adsorber. The benefits associated withthis technique include:

-   -   the NOx spike associated with the rich to lean transition of the        engine is avoided, and    -   the fuel penalty and hydrocarbon slip are reduced.

A second possible embodiment of the method involves using thein-cylinder transition from lean to rich operation to regenerate the NOxadsorber catalyst. During the time when the transition back to leanoperation is desired, a quantity of reductant is injected upstream ofthe regeneration catalyst. The injected reductant is used to maintain arich atmosphere over the NOx adsorber during the rich to lean transitionsuch that the high engine out NOx emissions are reduced over the NOxadsorber. This embodiment has similar benefits to that outlined for thefirst embodiment.

As noted above, the first region below line 950, region 951, andrepresented by low load and, in general, low speed, demonstrates theoperating range wherein in-cylinder regeneration is desired. As exhaustgas is cooler when the engine is operating under these conditions,in-cylinder generation of a rich environment with excess CO and some H₂is, generally, more efficient than would be the case if catalyst 42 (orclose coupled catalyst 74) were needed to generate the desirablecondition in-line for regeneration.

The region outside of region 952 with high speed or load and bounded byline 954 shows a range of torque and speed at which full flow of exhaustgas though the adsorber is preferred. Beyond this region to high speedsor torque or both as represented by lines 956, 958, 960, bypass flow isaccommodated as demonstrated and discussed above in relation to FIG. 3.

Note when this method of control of the aftertreatment system is usedfor a high-pressure direct injection engine the range of region 951tends to be larger, pushing line 950 higher on the load/speed plot ascompared to a spark ignited engine that uses fumigated fuels to operate.That is, the effective expansion ratio tends to be larger resulting incooler exhaust gas in general being expelled from the engine than is thecase for a spark-ignited engine.

Note also, in-cylinder regeneration as demonstrated in FIG. 6 need notrely on a by-pass flow during regeneration. That is, the high loadregion bounded by line 954 could be the extent of operation of theregeneration control map. Here three regions would exist forregeneration of a lean NOx adsorber. Low load strategy represented byregion 951, midrange load strategy represented by region 952 and highload strategy bounded by line 954 would represent the entireregeneration control map. Each region would use in-cylinder and inlineregeneration strategies as taught above with no need for by-pass.

As noted above, the regeneration cycle is dependant on the exhaust gastemperature. It is important that the exhaust gas introduced intocatalyst 42 have a temperature above a minimum temperature to ensurethat the catalyst is “lit-off” initially. An additional way ofcontrolling the regeneration process from the combustion chamber is tochoose a combustion strategy or combustion timing that ensures eitherrelatively late heat release, as might be the case with spark ignitedengines, or a delayed or second direct injection of fuel into thecombustion chamber late in the power stroke when regeneration isrequired. This can also reduce NOx levels with associated benefitsduring regeneration as a quantity of exhaust gas can be directed throughthe by-pass line without NOx treatment. A reduced NOx level has benefitshere. Other strategies are well known to persons skilled in the art.

As natural gas is, overwhelmingly, methane with a few additional heavierhydrocarbons, C2 and C3 hydrocarbons in general, the methane store 36can be the fuel storage tanks if the engine is fueled by natural gas.That is, methane store 36 can be a natural gas source such as the enginefuel tanks.

Also, valves 28 and 29 can be injectors that would directly injectmethane into exhaust line 22. An injector as the reductant flow controlwould provide greater control over the timing and quantity of methaneand, therefore, greater control over the regeneration cycle.

A metal substrate for carrying the catalyst is generally preferred,rather than, for example, a ceramic substrate, if the metal substrateimproves thermal response to catalyst 42. As noted above, the quickerthe thermal response the quicker the regeneration process can becompleted, thereby reducing the amount of untreated exhaust gas allowedto flow through by-pass line 12.

An additional embodiment of the aftertreatment system can include avalve for introducing methane downstream from an oxidation catalyst andupstream of a reformer with catalyst 42 comprising an oxidation catalystand reformer in series but not sharing a common interface. Flow ofmethane through such a downstream valve can be controlled in response tothe quantity of methane needed within the exhaust gas entering areformer. After the exhaust gas has passed through an oxidation catalystits properties are changed. There will be less oxygen within the gas andless methane. This is because oxidation of methane occurs within thecatalyst. This consumes oxygen. As methane serves to provide the sourcefor H₂ and CO, which are preferred components in the regenerationprocess (see reactions (4) and (5) above), the quantity of methaneneeded within the reformer is determined by the amount present withinthe exhaust stream upstream of the reformer. The amount of methanepreferred is determined by that present in the gases which are exitingthe oxidation catalyst and the H₂ and CO concentrations preferred inlight of this initial quantity of methane present, which is the methanenot oxidized within oxidation catalyst.

Once forced through the oxidation catalyst, the exhaust gas,supplemented with methane via a downstream valve, is forced through thereformer. The reformer utilizes the high temperature of exhaust gasheated in the oxidation catalyst and the combustion chamber to drivereformation of methane within the reformer in line 22 to provide H₂ andCO downstream from catalyst 42. This stream is directed into NOxadsorber 46 when H₂ and CO regenerate NOx adsorber 46.

An oxidation catalyst can be a component of catalyst 42, and can be anyoxidization catalyst suitable for oxidizing the exhaust gas to reducethe oxygen content. By way of example, a suitable oxidation catalyst canpromote the following reactions:C_(x)H_(y)+(x+(y/4))O₂(Pt)→xCO₂ +y/2H₂OC_(x)H_(y)+(x+(y/4))O₂(Pd)→xCO₂ +y/2H₂OC_(x)H_(y)+(x/2)O₂(Pd)→xCO+y/2H₂CO+½O₂→CO₂By way of example only, for the operating conditions known for thisapplication, a suitable washcoat formulation comprises Al₂O₃. Othersuitable washcoat formulations may also be used, as would be understoodby a person skilled in the art.

A reformer can be a component of catalyst 42, and reformers suitable forthis application are well known. The reformer is preferably suitable toconvert methane with water to CO and H₂. By way of example, the reformercan be a precious metal-based catalyst with washcoat materials includingAl₂O₃.

NOx adsorber 46 typically adsorbs and stores of NOx in the catalystwashcoat while operating under lean conditions and NO₂ can be releasedand reduced to N₂ under rich operating conditions when a regenerationmixture, that includes hydrogen and rich exhaust gas, is passed throughthe adsorber. As noted above, the following shows typical operation ofthe NOx adsorber under lean conditions:NO+½O₂(Pt)→NO₂XO+2NO₂+½O₂→X(NO₃)₂and under rich conditions:X(NO₃)₂→XO+2NO+ 3/2O₂X(NO₃)₂→XO+2NO₂+½O₂NO+CO(Rh)→½N₂+CO₂2NO₂+4H₂→N₂+4H₂Owhere X is provided in the washcoat and is typically an alkali (forexample, K, Na, Li, Ce), an alkaline earth (for example, Ba, Ca, Sr, Mg)or a rare earth (for example, La, Yt).

An inline external heater can be used to help light off catalyst 42 andpromote reformation and oxidation of exhaust gas during regeneration.While, it is preferable that the majority of heat is provided by theexhaust gas, things such as, by way of example and not limited to:

-   -   transient response,    -   efficiency considerations,    -   combustion strategies that utilize a quick heat release,    -   valve timing, or    -   cylinder design that takes advantage of a large expansion ratio,        can release exhaust gas that could benefit from such a heater in        order to initiate oxidation prior to or during regeneration.

A heat exchanger could direct a quantity of heat from the outlet ofcatalyst 42 or heat from gases unused after regeneration out of adsorber46 back through to a point along line 22 upstream of catalyst 42. Thiscould be used to help reduce the load on such heater after it initiallylights the catalyst off.

Note that for reforming, as noted above, steam is required in order togenerate H₂ and CO for regeneration. This need tends to be met asexhaust gas has sufficient quantities of water. However, if water levelsare low, a partial oxidation catalyst (POX) catalyst can be employed toreform the gas without the need for supplemental water: see reaction(4). Other reformers could be used as understood by a person skilled inthe art.

Steam is made more available by oxidizing methane as compared to otherhydrocarbons. This is an additional advantage in light of the above.

A further advantage can be realized if a fuel is used that combinesmethane and hydrogen as two major components. By way of example, naturalgas with 10 to 50% hydrogen might be appropriate as an engine fuel andappropriate for regeneration. Such a fuel could then be utilized in theembodiments discussed wherein the hydrogen introduced with the fuelprior to the oxidation catalyst could help to light off those catalystsand help to provide an exhaust gas environment with a lambda lessthan 1. Further, by providing a quantity of hydrogen into the exhauststream, the burden on catalyst 42 is reduced. Less reforming is requiredfor regeneration due to the presence of hydrogen in the injected fuel.

The method taught above for bypassing exhaust gas can also be used ifhydrogen is injected into the exhaust gas. Here, the regenerationstrategy is driven by a target regeneration flow through the NOxadsorber that would efficiently regenerate while limiting the associatedfuel penalty and release of untreated NOx and NOx slip duringregeneration. This is that much more beneficial if the engine is fueledby hydrogen, with the fuel providing a ready source of reductant, butthis method would be useful, as well, if an external reformer can beused. Further, use of a two-bed aftertreatment system, as discussedabove and demonstrated in FIG. 5, would be useful if hydrogen can bedirectly injected into the exhaust gas upstream of a NOx adsorber duringa regeneration cycle after determining a target regeneration flow.

A further embodiment of the invention includes an aftertreatment systemthat has an in-line particulate filter. Referring to FIG. 1, aparticulate filter may be disposed in line 22 near and downstream ofreformer 42. The particulate filter could then benefit from the excessheat generated in line 22 during regeneration that, upon reintroductionof oxygen to this line after completion of a regeneration cycle, wouldgenerate an exotherm across the particulate filter that could burn offsoot collected here extending or eliminating the regeneration cycles forthe particulate filter. Note, however, one issue which arises is thatthe effectiveness of the particulate filter in reducing particulatematter emission is lowered because part of the flow is diverted aroundthe particulate filter during the regeneration event.

In a further embodiment of the invention, a clean-up catalyst is used inthe aftertreatment system. Referring to FIG. 1, a clean-up catalyst (notshown) may be provided in line 22 beyond junction 48. The catalyst couldbe selected to reduce NOx (lean NOx catalyst) from bypass line 12(resulting during a regeneration cycle) or selected to remove reductant(CO or H₂) or other hydrocarbons that pass through the aftertreatmentsystem during regeneration or selected to remove hydrogen sulfide themight result during any desulfurization process of the adsorbers.

Whenever flow is referred to in this disclosure, it is the mass or molarflow, rate of the gas in question.

Exhaust gas recirculation (EGR) can also be utilized to help reduce NOxemissions during regeneration when a by-pass line is opened. IncreasedEGR rates during regeneration can reduce NOx generated in the combustionchamber resulting in less NOx flowing through by-pass line 12 and intothe atmosphere. Further, increases in EGR can also be used to reduce theconcentration in oxygen in the exhaust gas during regeneration,reducing, in turn the burden on the oxidation catalyst to reduce oxygenduring a regeneration cycle as well as reduce the amount of methaneneeded to burn off oxygen.

While methane is the preferred source for hydrogen, as would beunderstood by a person skilled in the art, other lighter hydrocarbons,generally, gaseous hydrocarbons, could be used including but not limitedto other gaseous hydrocarbons such as ethane, propane and butane.

While particular elements, embodiments and applications of the presentinvention have been shown and described, it will be understood, ofcourse, that the invention is not limited thereto since modificationsmay be made by those skilled in the art without departing from the scopeof the present disclosure, particularly in light of the foregoingteachings.

1. A method of regenerating a lean NOx adsorber, said lean NOx adsorberused to remove NOx from exhaust gas generated by combustion of a fuel ina combustion chamber of an operating internal combustion engine, saidmethod comprising: (a) determining a target regeneration flow of saidexhaust gas through said lean NOx adsorber; (b) directing a regenerationflow of said exhaust gas through said lean NOx adsorber, saidregeneration flow established by one of: bypassing a bypass flow of saidexhaust gas around said lean NOx adsorber when said target regenerationflow is less than exhaust gas flow from said engine, resulting in saidregeneration flow being substantially the same as said targetregeneration flow, and directing substantially all of said exhaust gasthrough said lean NOx adsorber when said target regeneration flow isgreater than said exhaust gas flow from said engine, said exhaust gasflow from said engine and said bypass flow established by reference toat least one of: (1) engine speed, (2) engine load, (3) engine intakemanifold temperature, (4) intake air mass flow, (5) engine inlet fuelflow, (6) engine intake manifold pressure, (7) measured engine exhaustgas flow, (8) exhaust gas temperature, and (9) exhaust gas pressure; (c)reacting, within said exhaust gas and upstream of said lean NOxadsorber, a reductant to maintain a lambda of said regeneration flow ofless than 1 across said lean NOx adsorber.
 2. The method of claim 1wherein said target regeneration flow is determined based, at least inpart, on an NOx concentration in said exhaust gas.
 3. The method ofclaim 1 wherein said reductant is hydrogen.
 4. The method of claim 1wherein said reductant is a hydrocarbon.
 5. The method of claim 4wherein said hydrocarbon comprises methane.
 6. The method of claim 5further comprising introducing hydrogen into said regeneration flow byreforming said methane within said exhaust gas upstream of said lean NOxadsorber.
 7. The method of claim 1 wherein said fuel and said reductantare interchangeable.
 8. The method of claim 1 wherein said bypass flowis directed through a second lean NOx adsorber.
 9. A method of operatingan internal combustion engine equipped with an aftertreatment system forremoving NOx from exhaust gas generated by combustion of a fuel in atleast one combustion chamber of said engine, said method comprising:during normal operation of said engine, directing all of said exhaustgas through a lean NOx adsorber; periodically regenerating said lean NOxadsorber by a regeneration cycle defined by a regeneration cycle starttime and a regeneration cycle end time, during said regeneration cycle:(a) determining a target regeneration flow of said exhaust gas throughsaid lean NOx adsorber, (b) directing a regeneration flow of saidexhaust gas through said lean NOx adsorber, said regeneration flowestablished by one of: bypassing a bypass flow of said exhaust gasaround said lean NOx adsorber when said target regeneration flow is lessthan exhaust gas flow from said engine, resulting in said regenerationflow being substantially the same as said target regeneration flow, anddirecting substantially all of said exhaust gas through said lean NOxadsorber when said target regeneration flow is greater than said exhaustgas flow from said engine, said exhaust gas flow from said engine andsaid bypass flow determined by reference to at least one of: (1) enginespeed, (2) engine load, (3) engine intake manifold temperature, (4)intake air mass flow, (5) engine inlet fuel flow, (6) engine intakemanifold pressure, (7) measured engine exhaust gas flow, (8) exhaust gastemperature, (9) exhaust gas pressure; (c) reacting, within the exhaustgas and upstream of the lean NOx adsorber, a reductant to maintain alambda of said regeneration flow of less than 1 across the lean NOxadsorber.
 10. The method of claim 9 wherein said reductant is hydrogen.11. The method of claim 9 wherein said reductant is a hydrocarbon. 12.The method of claim 11 wherein said hydrocarbon comprises methane. 13.The method of claim 12 further comprising introducing hydrogen into saidregeneration flow by reforming said methane within said exhaust gasupstream of said lean NOx adsorber.
 14. The method of claim 9 whereinsaid fuel and said reductant are interchangeable.
 15. The method ofclaim 12 wherein reacting of said hydrocarbon occurs within said exhaustgas prior to directing said bypass flow around said lean NOx adsorber.16. The method of claim 12 wherein reacting of said hydrocarbon occurswithin said regeneration flow.
 17. The method of claim 12 wherein saidhydrocarbon is directed into said exhaust gas by at least one of a valveand an injector.
 18. The method of claim 9 wherein said regenerationcycle end time is determined when said lambda of said regeneration flowdownstream of said lean NOx adsorber is below a pre-determined thresholdconcentration.
 19. The method of claim 9 wherein said regeneration cycleend time is determined when a concentration of said reductant downstreamof said lean NOx adsorber is above a pre-determined thresholdconcentration.
 20. The method of claim 13 wherein said regenerationcycle end time is determined when a concentration of at least one of COor H₂ downstream of said lean NOx adsorber is above a pre-determinedthreshold concentration.
 21. The method of claim 9 wherein saidregeneration flow is controlled by at least one valve.
 22. The method ofclaim 9 wherein said regeneration flow is controlled by a bypass valvein a bypass line and an exhaust valve in an exhaust line.
 23. The methodof claim 22 wherein said bypass valve is a variable control valve. 24.The method of claim 23 wherein said exhaust valve is a variable controlvalve.
 25. The method of claim 9 wherein said regeneration cycle starttime is determined when a NOx concentration within said exhaust gasdownstream of said lean NOx adsorber is in excess of a thresholdconcentration as compared to a NOx concentration out of said engine. 26.The method of claim 12 further comprising, when operating said engine ina predefined low load, low speed mode, wherein said lambda of saidexhaust gas is less than one as a result of combustion of said fuelwithin said combustion chamber.
 27. The method of claim 26 wherein saidengine is a direct injection engine.
 28. The method of claim 9 furthercomprising during said regeneration cycle directing said exhaust gasdownstream of said lean NOx adsorber through a clean-up catalyst. 29.The method of claim 28 wherein said clean-up catalyst removes NOx fromsaid exhaust gas.
 30. The method of claim 28 wherein said clean-upcatalyst removes reductant from said exhaust gas.
 31. The method ofclaim 28 wherein said clean-up catalyst removes hydrogen sulfide fromsaid exhaust gas.
 32. The method of claim 9 further comprising directingsaid exhaust gas through a particulate filter upstream of said lean NOxadsorber.
 33. The method of claim 9 wherein said bypass flow is directedthrough a second lean NOx adsorber.
 34. A method of operating a leanburn internal combustion engine equipped with an aftertreatment systemfor removing NOx from exhaust gas generated by combustion of a fuel inat least one combustion chamber of said engine, said method comprising:(a) during normal operation of said engine, directing all of saidexhaust gas, which results from combustion of a lean fuel mixture,through a lean NOx adsorber; (b) periodically regenerating said lean NOxadsorber using a predetermined regeneration strategy selected from oneof a high load strategy, a midrange load strategy and a low loadstrategy, said regeneration strategy causing said exhaust gas passthrough said lean NOx adsorber: during said high load strategy:reacting, upstream of said lean NOx adsorber, a reductant to maintain alambda of said exhaust gas of less than 1 across said NOx adsorber,during said low load strategy: transitioning from lean burn in saidnormal operation to rich burn of said fuel in said combustion chamber togenerate said exhaust gas wherein said lambda of said exhaust gas isless than 1; transitioning from lean burn in said normal operation torich burn of said fuel, with said combustion chamber in a richenvironment, to generate said exhaust gas wherein said lambda is lessthan 1, and reacting, upstream of said lean NOx adsorber, a reductant.35. An aftertreatment system for removing NOx from exhaust gas producedduring combustion of a fuel within a combustion chamber of an operatinginternal combustion engine, said aftertreatment system comprising: (a)an exhaust line for directing said exhaust gas from said engine, (b) alean NOx adsorber disposed in said exhaust line for removing said NOx,(c) a regeneration catalyst disposed in said exhaust line upstream ofsaid lean NOx adsorber, said catalyst capable of promoting oxidizing orreforming of a reductant, (d) a reductant line for delivering saidreductant from a reductant store to said exhaust line upstream of saidcatalyst, (e) a reductant flow control disposed in said reductant linefor controlling flow of said reductant into said exhaust line, (f) abypass line for directing said exhaust gas around said lean NOxadsorber, (g) a second lean NOx adsorber, (h) at least one bypass flowcontrol capable of controlling flow of said exhaust gas through saidbypass line, (i) a controller, (j) at least one sensor providing controlinformation to said controller, said controller capable of adjustingsaid at least one valve in response to said control information.
 36. Theaftertreatment system of claim 35 wherein said reductant is hydrogen.37. The aftertreatment system of claim 35 wherein said reductant is agaseous hydrocarbon, said catalyst capable of reducing said gaseoushydrocarbon to provide hydrogen with said exhaust gas.
 38. Theaftertreatment system of claim 35 wherein said catalyst is a reformer inseries with an oxidation catalyst.
 39. The aftertreatment system ofclaim 35 wherein said catalyst is a oxidation catalyst.
 40. Theaftertreatment system of claim 35 wherein said catalyst comprises anoxidation catalyst combined with a reformer.
 41. The aftertreatmentsystem of claim 35 further comprising a second close coupled catalystproximate to said engine for oxidizing said reductant when said exhaustgas proximate to said regeneration catalyst is at a temperature below apredetermined threshold temperature, said predetermined thresholdtemperature below which said catalyst is unable to efficiently promotereaction of said reductant.
 42. The aftertreatment system of claim 35further comprising an injector for injecting said reductant into saidexhaust line.
 43. The aftertreatment system of claim 35 wherein saidby-pass flow control is a valve.
 44. The aftertreatment system of claim35 wherein said reductant store is a fuel system of said engine.
 45. Theaftertreatment system of claim 35 further comprising a particulatefilter disposed in said exhaust line downstream of and proximate to saidregeneration catalyst.
 46. The aftertreatment system of claim 35 furthercomprising a second lean NOx adsorber in said bypass line.
 47. Theaftertreatment system of claim 46 further comprising a secondregeneration catalyst disposed in said bypass line upstream of saidsecond lean NOx adsorber.
 48. The aftertreatment system of claim 35further comprising a clean-up catalyst disposed downstream of said leanNOx adsorber, said clean-up catalyst capable of removing, from saidexhaust gas, at least one of: (a) said NOx, (b) said reductant, and (c)hydrogen sulfide.